Arrangement and a Method for Ventilation of a Space

ABSTRACT

The present invention relates to a ventilation arrangement for ventilation of a space ( 3 ) in a building, comprising a first chamber (A), a first connection ( 5   3 ) connecting the first chamber to an air supply duct ( 100   3 ) to allow forced supply of exterior air, a second connection ( 4 ) connection the first chamber (A 3 ) and the space ( 3 ), and a control system ( 30   3 ). The first chamber (A 3 ) has a third connection ( 6   3 ) allowing supply of recirculated air into the space ( 3 ), at least partly via the first chamber (A 3 ), a mechanical ventilation damper ( 12   3 ) to prevent air flowing out from the chamber through the air supply duct ( 100   3 ), a humidity sensing device ( 10   3 ), a first driven fan ( 8   3 ) arranged at the first connection ( 5   3 ) for supply of exterior air, a second driven fan ( 9   3 ) at the third connection ( 6   3 ) for forced recirculation of air. The control system ( 30   3 ) alternatingly drives the first and the second fan. The humidity sensor measures the relative humidity of exterior air and of recirculated air. Absolute humidities of exterior air and recirculated air are compared to determine which air has the lowest absolute humidity, which then is supplied to the space ( 3 ).

TECHNICAL FIELD

The present invention relates to an arrangement and to a method for ventilation of a space, particularly for reducing or preventing locally occurring humidity problems in the space, even more particularly for enabling humidity control.

BACKGROUND

Humidity problems are often produced in cold spaces ventilated by exterior air, often only due to the humidity of the surrounding air and the fact that non-heated spaces often are as cold as, or even colder than, their surroundings and they will therefore accumulate humidity to levels wherein mould, fungi and other microorganisms can grow. The problems are aggravated if, in addition thereto, there are internal humidity sources such as for example leakage of humid air into an attic space from the living area. These humidity problems both lead to health problems and to large economical damages.

In order to facilitate the presentation below, some definitions are here given:

Absolute humidity is, conceptually seen, the amount of water in vapour phase that a given volume of air contains, normally expressed in grams per cubic meter, i.e. the density of the water vapour.

Vapour pressure here means the partial pressure of the water vapour in air in agreement with the praxis of the construction field. It should thus be noted that the definition of vapour pressure is different from the established definition within for example organical chemistry (wherein the vapour pressure instead means the pressure at which the evaporation of a substance is in equilibrium with its liquid and solid phases at a certain given temperature).

The saturation vapour content at a certain temperature is the amount of water in vapour phase that air maximally can contain before the water vapour starts to condensate at a surface or starts forming particles in the air.

The relative humidity is the relation between current absolute humidity and the saturation vapour content at the current temperature. This is at a given temperature numerically the same as the relation between the vapour pressure and the saturation vapour pressure, cf. for example Nevander, Elmarsson “Fukthandbok: Praktik och Teori”, Svensk Byggtjänst 1994.

An arrangement has been described (WO 2009/038534 A1) by means of which a crawl space is ventilated with outdoor air only when the absolute humidity is higher indoors than outdoors. An arrangement of this type is of course just as applicable at a cool attic space. According to the document it is suggested that such an arrangement is provided with a heating element which is triggered depending on the risk for mould growth and an attic or a crawl space is protected from damages caused by mould, (Bergsten 2006). The technique has been found to function well, particularly the combined solution comprising both absolute humidity controlled ventilation and heating, in those cases where the ventilation is not enough to obtain a climate which is mouldsafe. In connection with the fact that there, as mentioned above, also may be internal humidity sources, such as for example leakage of humid air into an attic space from the living area, it has sometimes been shown to be difficult to, despite the technique described in the cited document, avoid spotwise damages as a consequence of a locally occurring too high absolute humidity, which, in spite of the arrangement, may provide conditions such that for example growth of mould and fungi is promoted. In order to eliminate also this problem an arrangement has been constructed, which in one unit provides all of the so far discussed solution in a package, and which furthermore contains a fan for circulation of the air contained in the space also when there is no ventilation with exterior air. The mixing of air that thereby is obtained could in principle eliminate the above mentioned problems efficiently.

However, the control of when ventilation with exterior air is to take place or not is performed, as earlier described, using information from humidity sensors, wherein the respective humidity is measured by means of a sensor at the inside and a sensor at the outside.

It is customary to somewhat carelessly use the expression humidity sensor. Even if this is a functionally descriptive term, it actually does not say anything about what is actually measured. However, normally the temperature and the relative humidity, H_(rel), are measured, which values used together give the absolute humidity.

The relations are as follows:

H _(rel)=absolute humidity/saturation vapour content (applicable as a general rule).

Since the saturation vapour content is a temperature dependent quantity it is available as a table value. Reconstruction of the above given relation gives:

Absolute humidity=H _(rel)×saturation vapour content.

Humidity sensors can in other words be described as something that measures the temperature and the relative humidity and something that, in some cases, from these values, extracts a signal corresponding to the absolute humidity.

A property of humidity sensors is that they although having a long term stable precision, they tend to drift in accuracy over time. The effect thereof is that previous systems either have required a regular calibration or their functioning has been poorer than what would have been possible with accurately calibrated sensors. So far it has not been possible to provide a ventilation arrangement which is long term stable, which allows ventilation and controlling of the humidity which is efficient and reliable and which functions well also when/if the accuracy of used humidity sensors is deteriorated with time. It has also been impossible to provide an arrangement which satisfactorily can handle situations with locally occurring humidity and prevent local damages due to humidity.

US 2003/0181158 describes a control system for a HVAC system (a heating, ventilation and air conditioning system) wherein the air is recirculated to assure that heating of air and cooling of air becomes more efficient, and wherein outdoor air is mixed in to the extent needed to meet requirements on fresh air. The arrangement described in this document has as an object to cool, heat and ventilate a space in a cost efficient manner. The arrangement controlled by a control system in the above mentioned document operates with two motorized, driven, ventilation dampers and a fan. A certain amount of outdoor air is always mixed in, in the document it is mentioned between 5-30%. However, it is difficult to prevent (local) damages, e.g. due to locally produced humidity, with such an arrangement and due to the construction of the arrangement it may be difficult to provide an optimal rotation of the air and also a drop in pressure which is as low as possible. It is also an expensive arrangement depending on heating and cooling needs and it is difficult to mount and install. It is a complicated arrangement normally requiring specialist installation. In particular it is not suitable for e.g. spaces which normally are not or should not be heated, e.g. attics, basements, but even if the spaces are heated, it does not solve the stated problems. From a maintenance point of view the arrangement is also demanding.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide an improved arrangement and a method respectively for ventilation of a space. It is particularly an object to provide an arrangement and a method respectively through which the humidity sensing errors of a humidity sensor can be reduced or compensated for. Particularly it is an object to provide a stand alone arrangement, i.e. an arrangement which is not dependent on any air condition system with heating and cooling.

It is also an object of the invention to provide an arrangement and a method through which ventilation and measurements of humidity can be carried out in such a way that the influence of drift in accuracy at humidity sensing can be eliminated.

It is a particular object of the invention to provide an arrangement which is reliable, long-term stable, robust, cheap, easy to fabricate, use and install, also for the layman. Another particular object to provide an arrangement through which the risk for damages due to fungi, mould and microbial growth resulting from (local) humidity can be reduced. Another particular object is to provide an arrangement which can reduce the risk for damages due to local, spotwise occurring, high humidity, e.g. due to internal humidity sources, or intruding humidity from the outside, e.g. due to rain, snow etc.

It is also an object to provide a ventilation arrangement which is less dependent on a high actual, current accuracy of humidity sensors than known arrangements. It is also an object to provide a ventilation method through which one or more of the above mentioned objects can be achieved.

A most particular object is to provide a stand-alone arrangement for ventilation preventing humidity problems in a normally not heated space, or an only sparsely heated space, but alternatively also for a normally heated space.

Therefore an arrangement as initially referred to is provided which has the characterizing features of claim 1. A method having the characterizing features of claim 15 is therefore also provided. Advantageous embodiments are given by the features of the appended sub-claims.

According to the invention a humidity sensor, which is connected to a control system, is so arranged or located in a chamber that it can sense the humidity in air provided via en opening, outdoor air and recirculated, interior, air. The humidity sensor according to some embodiment senses the relative humidities, in which case the absolute humidity has to be established, e.g. using table values or approximations, by the sensor or by the control system. In other embodiments the sensor measures absolute humidity or water vapour pressure. The absolute humidity values of the exterior air and the recirculated air are compared, and the control system controlling ventilation (air supply from outside) and recirculation makes sure that always the air with the lowest absolute humidity (or water vapour pressure) is provided to the space. The humidity (relative humidity or water vapour pressure or absolute humidity) from the opening/openings (outdoor air or recirculated air) is measured by one and the same sensor (connected to, or also comprising a temperature sensor), which has a consequence that a possible drift in accuracy of the sensor will be balanced when the values are compared, since the error is the same for exterior air and for recirculated air. It is an advantage of the invention that the arrangement is robust, that it easily can be so arranged that the air actually is rotated, circulated, properly, it is easy to mount, e.g. over a gable vent, without having to consider e.g. air flow resistance in the valve openings and if it can be used as a stand-alone unit, and can be mounted by a layman.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will in the following be further described, in a non-limiting manner, and with reference to the accompanying drawings, in which:

FIG. 1 shows a first schematical block diagram of an arrangement according to the invention,

FIG. 2 is a schematical block diagram of a second embodiment of an arrangement according to the invention,

FIG. 3 is a schematical block diagram of a third embodiment of an arrangement according to the invention,

FIG. 4A is a schematical view of an arrangement according to the invention with two chambers,

FIG. 4B is a view similar to that of FIG. 4A for a slightly different embodiment, and

FIG. 5 is a flow diagram describing a method for ventilation of a space according to one embodiment of the invention.

DETAILED DESCRIPTION

With reference to FIG. 1 an arrangement according to the invention is shown for a first implementation. The arrangement here comprises one chamber, here called a first chamber A₃ with two from one another separate openings, a first opening and a second opening 4 ₃. The first opening is via a channel 100 ₃ and a first connection 5 ₃ connected to outdoor air and to a space 3 to be ventilated/conditioned respectively via a connection 6 ₃. The second opening or connection 4 ₃ is provided at the air supply side of the space 3. Thus, either fresh outdoor air or recirculated indoor air from the space 3 can be supplied to space 3 from the chamber A₃ via the second connection 4 ₃. Which air that actually is supplied depends on which of (possibly both) of any at the first opening provided connections, either for outside air, 5 ₃ or for recirculated air, 6 ₃, is in operation. A first fan 8 ₃ at connection 5 ₃ is arranged to provide for supply of outdoor air whereas a second fan 9 ₃ is arranged to supply recirculated air and at any moment at least one of the fans is driven or active.

A reverse ventilation damper 12 ₃, which also may be a driven motorized valve, will at any moment prevent that air flows out via the channel 100 ₃ through which, as stated above, fresh outdoor air can be supplied.

At the beginning of the second opening or connection 4 ₃ a humidity sensor 10 ₃ (it is here supposed that it also comprises a temperature measuring functionality; a temperature sensor is alternatively provided as a separate sensor) is arranged, which in turn is connected to a control system 30 ₃ which is arranged to, by in an alternating manner driving the first fan 8 ₃ and the second fan 9 ₃, establish whether it is the outdoor air or the indoor air from the space that has the highest absolute humidity or water vapour pressure. When it has been established which air is the driest, or shows the lowest absolute humidity value or lowest water vapour pressure, this air is used as ventilation, or recirculation, air to be supplied to the space 3. The establishment of the relation between the said absolute humidity levels (or water vapour pressure) takes place at regular intervals, for example once an hour, whereupon a change to ventilation or recirculation depending on the outcome of the comparison will take place. This procedure runs thereafter without interruption. It should be clear that the establishment of the relationship between e.g. the absolute humidities, or the comparing of the absolute humidities, can be done also at more or less regular intervals and it is of course not limited to being done once per hour but it can be done with a lower or a higher frequency as well. The absolute humidity is established by the control system 30 ₃ having access to tabulated values from an information holding means 40 ₃, which may be comprised in the control system, or be external. Alternatively the sensor is capable of, from measured relative humidity establish absolute humidity (or water vapour pressure). Still further a sensor could be used that actually measures absolute humidity or water vapour pressure.

FIG. 2 shows an alternative implementation of an arrangement. It here comprises two chambers, a first chamber A₄ and a second chamber B₄, and also a heating element 20 ₄. The first chamber A₄ is connected to interior or indoor air of the space 3 via a second connection 4 ₄ and also to exterior or outdoor air via a first connection 5 ₄, and furthermore to the second chamber B₄ via a connection 6 ₄. The second chamber B₄ is connected to indoor or interior air via a connection 7 ₄ (and to the first chamber A₄ via the connection 6 ₄). A first fan 8 ₄ is arranged at the entry to the first chamber A₄ and it is capable of driving air from the outside, outdoor air, via the connection 5 ₄, and via a reverse ventilation damper 12 ₄ to the indoor air connection 4 ₄. The connection 6 ₄ between the first chamber and the second chamber is located after the reverse ventilation damper 12 ₄ and the first chamber A₄. The reverse ventilation damper 12 ₄ is so arranged that it opens when the first fan 8 ₄ in the first chamber A₄ is active, and is closed when the first fan 8 ₄ in the first chamber is inactive, and when a second fan 9 ₄ in the second chamber B₄, at the connection of the second chamber B₄ to indoor air, is active.

In order to assure that the reverse ventilation damper 12 ₄ always opens when it should, preferably the second fan 9 ₄ is so arranged that it gives an overpressure which is the same as or lower than a pressure produced by the first fan 8 ₄. The second fan 9 ₄ is so arranged in the second chamber B₄ that it drives air via the interior air connection 7 ₄ to the second chamber into the first chamber. In a particularly advantageous embodiment of the invention a heating element 20 ₄ is located close to the connection 6 ₄ between the first and second chambers. The arrangement according to FIG. 2 also comprises a control system 30 ₄ and a humidity sensor 10 ₄ (as discussed with reference to FIG. 1, including also a temperature sensor, or a temperature sensor being provided for separately), which in this embodiment are located in the first chamber A₄ and close to the connection 4 ₃ between the first and second chambers.

In a particularly advantageous embodiment the control system 30 ₄ may comprise or have access to saturation vapour content data for example in a storing or holding means 40 ₄ arranged in the control system 30 ₄ although this is not necessary for the functioning of the inventive concept if absolute humidity or water vapour pressure can be measured or obtained e.g. by the sensor.

Particularly the humidity sensor 10 ₄ is a sensor for measuring both temperature and relative humidity. It is preferably controlled by the control system 30 ₄. If a heating arrangement 20 ₄ is provided, both the humidity sensor and the heating arrangement are connected to the control system which preferably controls the heating element, switching it on, switching it off, or increasing and decreasing its heating power.

In embodiments where a heating arrangement 20 ₄ actually is provided to heat up the air, the humidity sensor 10 ₄ is arranged after the heating arrangement in the direction of the flow of the air in some appropriate manner.

FIG. 3 shows an alternative to the implementation given in FIG. 2 and similar means and components are given the corresponding reference signs but with index 5. It is here supposed that the connection between the first chamber A₅ and the second chamber B₅ has been rotated about 90° clockwise whereas the connection 4 ₅ to the space 3, to which air is forced either from the outside or as recirculated air from the second chamber B₅, has been moved downwards as compared to in FIG. 2. Such an embodiment can be of advantage from an air flow point of view since the flows from the two chambers will be well balanced.

The control system 30 ₅ is arranged substantially as in FIG. 2, and may communicate with, or comprise, an information holding means 40 ₅ comprising information on saturation vapour content of air for different temperatures. The humidity sensing device 10 ₅ is here arranged at the entry, beginning, of connection 4 ₅ to space 3.

In this embodiment the control system 30 ₅ uses measurements from the humidity sensing device 10 ₅, establishes the absolute humidity or water vapour pressure and controls the fans 8 ₅ and 9 ₅ and the heating arrangement 20 ₅ as will be described below.

The control system 30 ₅ provides for taking of periodical measurements on the outdoor air by activating (switching on) the fan 8 ₅ and by deactivating (switching off) the fan 9 ₅ and the heating arrangement 20 ₅, and, after the sensing device has settled, (become stable) or after the lapse of a certain period of time, via the sensing device one or several consecutive measurements are carried out in order to establish the water vapour pressure and/or absolute humidity of the exterior, outdoor air.

Further, under control of the control system, periodical measurements on indoor air are carried out while deactivating fan 8 ₅ and the heating arrangement 20 ₅ (if it was on) and activating fan 9 ₅ and, after the sensor has settled, taking one or more consecutive measurements to establish water vapour pressure and/or absolute humidity in indoor air. One of, or both, fans 8 ₅, 9 ₅ are always active. As discussed above, the relative humidities may be measured and the absolute humidity/water vapour pressure established by the control system.

The control system 30 ₅ drives, in periods between measurements, the fan 8 ₅ to blow air (draw air in) from the outside when the vapour pressure or absolute humidity is lower in outdoor air than in indoor air, in one embodiment in such a manner that the operation speed is kept at a level lower than a normal operation level when the difference in water vapour pressure or absolute humidity is low, and the speed of rotation is kept at a higher or normal operation level when the difference in water vapour pressure or absolute humidity is large, possibly with an off-set which starts the fan only if a certain smallest difference in vapour pressure or absolute humidity is established.

The control system 30 ₅ controls, between the measurement periods, i.e. when one or more measurements are carried out, the fan 9 ₅ so that it forces indoor air from the inside into the first chamber A₅ and via its connection to the indoor air, back to the indoor air in space 3 when the fan 8 ₅ is inactive or when the heating arrangement 20 ₅ is active (which thus can be at same time as the fan 8 ₅ is active). The control system 30 ₅ controls, between the measurement periods, the heating arrangement so that it through a trigger point is active when the relative humidity of the indoor air exceeds said trigger point. The trigger point can be a fixed value on relative humidity or depend on the temperature and possibly also the duration (in time) that the indoor air has been more humid or had a higher relative humidity than the value of the trigger point, so that the trigger point will follow the risk for microbial growth which, as such, also is temperature and time dependent.

Since the difference in for example absolute humidity is calculated as a difference between two values based on two measurement values measured by one and the same sensor, an error which depends on the sensor accuracy will to a large extent be compensated for any error by itself as long as the sensor has a good precision. Thereby it can be avoided that drift in sensor accuracy affects the drying action that the ventilation arrangement is intended to have.

The switching on/off of the heating element 20 ₅ is done towards an absolute value on the relative humidity and a drift in accuracy as far as a measurement thereof is concerned, will affect the trigger point, which can have as a result an unnecessarily high energy consumption or a poor function.

This can, for such an arrangement, in advantageous implementations be prevented by compensating for measurement errors as mentioned above by means of an extended functionality of the control system according to the invention whereby the relative humidity and the temperature are measured when the fan 8 ₅ and the heating arrangement 20 ₅ are switched off and, here, the fan 9 ₅ is driven, whereby one or more measurements are performed after the lapse of the time that it takes for a sensing device 10 ₅ to acclimatize and become stable, giving a first relative humidity at a first temperature. Then the heating element 20 ₅ is switched on, activated, and after the time it takes for the sensor to become stable so that the temperature of the air where the sensing device 10 ₅ is arranged has been increased considerably as compared to the temperature in the space 3, for example between 10° C. and 70° C. higher, particularly between 30° C.-60° C. higher, e.g. at a temperature about 50° C. higher, or, at a temperature of about 50° C., one or more consecutive measurements are performed on temperature and relative humidity at the said second temperature. Then a nominal humidity is calculated as the absolute humidity (e.g. established by the control system using tabulated values) at the first temperature divided by the saturation content value of a second measurement at the second temperature as discussed above.

This value on nominal relative humidity will have a value which has been reduced in relation to the difference in saturation vapour content at a first temperature and at a second temperature. Then a measured relative humidity is calibrated towards the nominal relative humidity and the calibration value (the difference between the nominal relative humidity and the measured relative humidity) is saved for future adjustments of obtained measurement values. (Even if this is described with reference to FIG. 3, it should be clear that it is of course also applicable for arrangements as in FIG. 2.) The following example shows such a case in practice.

The system measures the relative humidity, H_(rel), with an error of +/−10% to 90% H_(rel) at the temperature 0° C. The absolute humidity will then be the relative humidity value multiplied by the saturation vapour content value at the temperature in question, which in the case of 0° C. is 4.86 g/m³, thus 90%×4.86g/m³=4.374 g/m³ (+/−10%). Then the heating element is activated whereupon the temperature is increased to 50° C. The saturation vapour content at 50° C. is about 83.11 g/m³. The nominal relative humidity is the relative humidity divided by the saturation content value which makes 4.374 g/m³/83.11 g/m³=5.26%. If the error in the measurements made by the sensing device was 10% earlier, the hence calculated nominal H_(rel) error has an error which is 10%×4.86/83.11=0.58%. A calibration towards the nominal RH_(nom) thus reduces the error from 10% to 0.58%. If thus the established measurement error (the difference between measured H_(rel) and nominal H_(rel)(RH_(nom)) was very large, the procedure can be repeated after the first calibration so that the measurement error can be still further reduced.

The error compensation, that in advantageous implementations may be used with an arrangement according to the invention, which thus has to include a heating element, will here be explained more in detail, (for exemplifying reasons only with reference to FIG. 3):

The humidity sensor 10 ₅ measures the relative humidity in air recirculated from the space at a first, lower, temperature giving a first relative humidity value H_(rel)(T₁). In one particular embodiment this is done before the heating arrangement is switched on or activated, i.e. in the absence of heating. In alternative embodiments the heating arrangement may already have been switched on, or it comprises an activated heating arrangement for heating the space, in which case the second temperature normally is not so much higher than the first temperature leading to more iterations. Particularly the space is a space which normally is not heated. However, the invention is also applicable if the space is heated by some other heating arrangement.

The first relative humidity value and the first temperature value are provided to the control system, which (alternatively this function is performed in the sensing device itself, which also alternatively can have access to the storing means), for the first temperature T₁, accesses an information holding means 40 ₅ to find the saturation vapour content value V_(sat)(T₁) representing the saturation vapour content at said first temperature, and thereupon calculates a first absolute humidity value V_(c)(T₁) by multiplying the first saturation content value V_(sat)(T₁) by the first relative humidity value H_(rel)(T₁). Then the control system is adapted to switch on or increase the power of the heating arrangement 20 ₅ preferably until a considerably higher temperature is reached; it may be controlled or monitored continuously to see if/when the temperature value becomes stable, or it can be monitored at intervals in any other appropriate manner.

Preferably the temperature is increased by 10° C.-70° C., preferably between 30° C.-60° C. as referred to above. Alternatively it is heated to a temperature between 20° C. and 70° C., preferably between 40° C. and 60° C., e.g. about 50° C. is reached. Then, for this temperature, which is called a second temperature, T₂, also the relative humidity is measured by the humidity sensor 10 ₅ thus providing a second relative humidity value H_(rel)(T₂). Then, by means of the control system 30 ₅, a tabulated or approximated saturation vapour content value V_(sat)(T₂) representing the saturation vapour content at said second temperature is found or established. The control means are then adapted to calculate a nominal relative humidity value RH_(nom) by dividing the first absolute humidity value V_(c)(T₁) by the second saturated vapour content value V_(sat)(T₂) and to subsequently establish the difference ΔRH between the second measured relative humidity value and the said nominal relative humidity value.

This difference value ΔRH is then used to compensate for the error in subsequent measurement results on relative humidity performed by the humidity sensor. This procedure can also be repeated by using a compensated first relative humidity value as the first relative humidity value and performing the above described iterations one or more times. Normally, if the temperature difference is high, the number of iterations that need to be done is lower than if only a small temperature difference between T₁ and T₂ is used. If, and how many times, the iteration procedure is repeated, depends also on the accuracy that is needed.

According to the general concept of the invention an air drying effect is obtained in the interior space since the space is recirculated with outdoor air when the ventilation has an drying effect on the air, otherwise not; in such a way the space will be dried by means of outdoor air when the supplied outdoor air contains less humidity than the air that is ventilated out of the space; through the heating element the air temperature can be increased, and thereby the humidity be decreased, as the humidity of the air otherwise would have allowed growth of mould; and since always at least one fan is active, the air will always be circulated, rotated, in the interior space which reduces the risk for local leakage or local temperature differences providing locally increased humidity values.

Since the same sensor or sensors is/are used to establish the vapour pressure or the absolute humidity in interior air as well as exterior air, it can be avoided that drift in the accuracy of sensors provide errors in measurement results—a sensor which produces erroneous measurements due to drift will deliver results with the same error for outdoor air as for indoor air and in the difference calculation it will substantially be balanced out since the absolute humidity and vapour pressure around a certain temperature can be approximated by a linear function. By using a heating element to create a nominal relative humidity with a considerably lower error than the measurement value provided by a sensor for relative humidity, the error of the sensor can continuously be reduced through calibration against the nominal relative humidity.

The invention can be varied in different manners. For example the second chamber can be reduced only to the fan housing of the second fan, the control system can be provided in the first or in the second chamber or at the outside of both these chambers etc.

In order to still further reduce the measurement errors the expansion of air with temperature can be also be taken into consideration. The error produced thereby is very low but results from the fact that the air volume that passes the chamber or the heating element or into the heating element is not the same as the air volume that exits therefrom. When air is heated from for example 0° C. to 50° C., the volume increases with about 16.7%. This is below exemplified by means of an example.

It is supposed that the air temperature is 0° C., the relative humidity is 70%. The absolute humidity is the saturation vapour content value, here 86 g/m³ multiplied by H_(rel), 0.7, which gives 4.86×0.7=3.4 g/m³.

If the temperature is raised by 50° C., the air will expand by about 16.7%. The vapour content will then be 3.4/1.167=2.9 g/m³ and the saturated vapour content value is 83 g/m³. The real nominal relative humidity will then be 2.9/83=3.5% instead of 3.4/83=4.1%. It should however be noted that for most practical implementations this error is so low that it will be nearly insignificant but it can be compensated for when needed.

FIGS. 4A, 4B show two exemplary implementations of arrangements with two chambers.

FIG. 5 is a flow diagram schematically describing the procedure according to one implementation of the present invention. It is supposed that, 100, supply of exterior air is enabled by means of a first fan at a first connection to a (first) chamber whereby the air flows through said chamber and via a second connection to the space to be ventilated. It is also supposed, 102, that supply of recirculated air into the space is enabled, so that the air at least partly flows via the chamber and trough a second connection by means of a second fan. Air is prevented from flowing out of the first chamber through the exterior air supply opening by use of a mechanical reverse ventilation damper, (102), (steps 101, 102, 103 are not sequential steps). It is hence supposed that, when the first fan is activated, the second fan is inactivated, the relative humidity and temperature of exterior air can be measured using a humidity sensor, 103. Then, the first fan is inactivated, the second fan is activated and the relative humidity and temperature of recirculated air can instead be measured. For this purpose the same humidity sensor is used as for measuring the outdoor air, 104. Steps 103, 104 can be performed in any order.

The different steps can be carried out at discrete time intervals or as a substantially continuous procedure. When there have been carried out measurements of the exterior air as well as the recirculated air, and the corresponding absolute humidity values, or water vapour pressure values, have been established, unless directly measured, it is determined if the exterior air or the recirculated air has the lowest humidity, 105. Then the space to be ventilated is supplied with the air, i.e. the exterior outdoor air or the recirculated air, that has the lowest humidity, 106.

The invention is of course not be limited to the explicitly illustrated embodiments, but can be varied in a number of ways within the scope of the appended claims.

The different steps can be carried out at discrete time intervals or as a substantially continuous procedure. When there have been carried out measurements of the exterior air as well as the recirculated air, and the corresponding absolute humidity values, or water vapour pressure values, have been established, unless directly measured, it is determined if the exterior air or the recirculated air has the lowest humidity, 105. Then the space to be ventilated is supplied with the air, i.e. the exterior outdoor air or the recirculated air, that has the lowest humidity, 106.

The invention is of course not be limited to the explicitly illustrated embodiments, but can be varied in a number of ways within the scope of the appended claims. 

1.-17. (canceled)
 18. An arrangement for ventilating a space in a building, comprising: a first chamber; a first connection connecting the first chamber to an air supply duct configured for forced supply of exterior air; a mechanical ventilation damper configured for preventing air from flowing out from the first chamber through the air supply duct; a first fan arranged at the first connection for supplying exterior air; a second connection connecting the first chamber and the space; a third connection to the first chamber configured for forced supply of recirculated air into the space, at least partly via the first chamber; a second fan arranged at the third connection configured for forced recirculation of air; a humidity sensing device configured for measuring a humidity of exterior air supplied via the air supply duct and a humidity of recirculated air from the space; and a control system configured for driving, in an alternating manner, either the first fan or the second fan, wherein the control system is configured for comparing the absolute humidity of exterior air and the absolute humidity of recirculated air to determine whether exterior air or recirculated air has the lower absolute humidity, and to controllably supply the air with the lower humidity to the space.
 19. The arrangement of claim 18, further comprising a heating element configured for control by the control system and located after the second fan in a direction in which the recirculated air and exterior air flow.
 20. The arrangement of claim 18, wherein the first connection includes a recirculation connection for recirculated air forced into the space via the first chamber by operation of the second fan.
 21. The arrangement of claim 19, further comprising a second chamber in communication with the first chamber via a recirculation connection for recirculated air forced into the space via the first chamber and the second connection.
 22. The arrangement of claim 21, wherein the second fan and the heating element are arranged in the second chamber.
 23. The arrangement of claim 22, wherein the heating element is arranged adjacent to the recirculation connection between the second and first chambers in a flow direction of recirculated air via the second chamber.
 24. The arrangement of claim 18, wherein the humidity sensing device comprises a sensor for sensing relative humidity and a temperature sensor, both connected to the control system.
 25. The arrangement of claim 24, wherein the humidity sensing device is configured for measuring relative humidity and temperature of exterior air and of recirculated air, and the control system is configured to determine respective saturation vapor content values corresponding to the respective temperatures and to generate respective absolute humidity values based on the respective saturation vapor content values and temperatures by at least multiplying a measured respective relative humidity by the respective saturation vapor content value for the respective measured temperature.
 26. The arrangement of claim 25, wherein at least one of the control system and the humidity sensing device comprises a memory configured with stored values of saturation vapor content at different temperatures; the humidity sensing device is configured for measuring relative humidity at a first temperature T₁ and for providing a first relative humidity value H_(re1)(T₁) at the first temperature T₁; the control system is configured to generate a first absolute humidity value V_(c)(T₁) for the first temperature T₁ by at least finding a first stored saturation vapor content value V_(sat)(T₁) and multiplying the first stored saturation vapor content value by the first relative humidity value; the humidity sensing device is configured for measuring relative humidity at a second temperature T₂ higher than the first temperature T₁ and for providing a second relative humidity value H_(re1)(T₂) at the second temperature; the control system is configured to generate a second vapor content value H_(sat)(T₂) at the second temperature T₂, to generate a nominal relative humidity value RH_(nom) by at least dividing the first absolute humidity value V_(c)(T₁) by the second saturation vapor content value V_(sat)(T₂), to determine a difference ΔRH between the second relative humidity value H_(re1)(T₂) and the nominal relative humidity value RH_(nom), and to adjust subsequent relative humidity values of the humidity sensing device based on the difference ΔRH, thereby providing compensated relative humidity values.
 27. The arrangement of claim 26, wherein the control system is configured to use compensated relative humidity values as the first and second relative humidity values and to determine the difference ΔRH based on the compensated relative humidity values, and to repeat the use a plurality of times, thereby increasing accuracy of the relative humidity values and compensated relative humidity values.
 28. The arrangement of claim 19, wherein the heating element is arranged in the first chamber or in a connection receiving recirculated air from the space.
 29. The arrangement of claim 18, wherein the humidity sensing device is configured for measuring temperature and relative humidity of air in or from an unheated space.
 30. The arrangement of claim 18, wherein the humidity sensing device is configured for measuring temperature and relative humidity of air in or from a heated space.
 31. The arrangement of claim 26, wherein the second temperature is about 10° C.-70° C. higher than the first temperature.
 32. The arrangement of claim 26, wherein the control system is configured for further adjusting the subsequent relative humidity values for temperature-dependent expansion of air.
 33. The arrangement of claim 18, wherein the humidity sensing device comprises a sensor for sensing absolute humidity or water vapor pressure.
 34. A method of ventilating a space in a building, comprising: enabling supply of exterior air by a first fan at a first connection communicating with an air supply duct to a first chamber, through the first chamber, and via a second connection into the space; enabling forced supply of recirculated air by a second fan into the space at least partly via the first chamber; preventing air from flowing out from the first chamber via the air supply duct by a mechanical reverse ventilation damper; alternately driving the first and second fans; measuring a humidity of the exterior air with the second fan inactive and a humidity of the recirculated air with the first fan inactive; comparing absolute humidity values of the exterior air with absolute humidity values of the recirculated air to determine which values are lower; and controlling at least one of the first and second fans to ventilate the space using air having the lower humidity value.
 35. The method of claim 34, further comprising measuring respective temperatures of the exterior air and the recirculated air.
 36. The method of claim 35, wherein measuring the humidities of the exterior and recirculated airs include measuring relative humidities of the exterior and recirculated airs, and the method further comprises determining respective saturation vapor content values corresponding to respective temperatures of the exterior and recirculated airs, and generating respective absolute humidity values based on the respective saturation vapor content values and temperatures by at least multiplying a measured respective relative humidity by the respective saturation vapor content value for the respective measured temperature.
 37. The method of claim 36, wherein relative humidities are measured at a first temperature T₁ and first relative humidity values H_(re1)(T₁) at the first temperature T₁ are provided; first absolute humidity values V_(c)(T₁) for the first temperature T₁ are generated by at least finding a first stored saturation vapor content value V_(sat)(T₁) and multiplying the first stored saturation vapor content value by the first relative humidity values; relative humidities are measured at a second temperature T₂ higher than the first temperature T₁ and second relative humidity values H_(re1)(T₂) at the second temperature are provided; second vapor content values H_(sat)(T₂) at the second temperature T₂ are generated; nominal relative humidity values RH_(nom) are generated by at least dividing the first absolute humidity values V_(c)(T₁) by the second saturation vapor content values V_(sat)(T₂); differences ΔRH between second relative humidity values H_(re1)(T₂) and nominal relative humidity values RH_(nom) are generated; and subsequent relative humidity values are adjusted based on the differences ΔRH, thereby providing compensated relative humidity values. 